Metalliding process

ABSTRACT

THE DISCLOSURE TEACHES NOVEL COPPER BASE ALLOYS HAVING IMPROVED TOUGHNESS AND OUTSTANDING RESISTANCE TO STRESS CORROSION. THE COPPER ALLOYS CONTAIN, IN WEIGHT PERCENTPRODUCE A BARRIER LAYER OF METALLIC CARBIDE AND BORONIZED TO PRODUCE A BARRIER LAYER OF METALLIC BORIDES. THIS ABSTRACT IS NEITHER INTENDED TO DEFINE THE INVENTION OF THE APPLICATION, WICH, OF COURSE, IS MEASURED BY THE CLAIMS, NOR IS IT INTENDED TO BE LIMITING AS TO THE SCOPE OF THE INVENTION IN ANY WAY.

United States Patent 01 Lfice 3,824,134 Patented July 16, 1974 3,824,134METALLIDING PROCESS Maynard R. Chance, Houston, Tex., assignor toThornhill- Craver Company, Houston, Tex. No Drawing. Filed Oct. 12,1971, Ser. No. 188,524 Int. Cl. C23c 11/12 US. Cl. 148-6 2 ClaimsABSTRACT OF THE DISCLOSURE A metalliding process for producing extremelyhard metallic boride surface layers on steel bodies wherein the steel iscarburized and treated with a nonferrous metal to produce a barrierlayer of metallic carbide and boronized to produce the surface layer ofmetallic borides. This abstract is neither intended to define theinvention of the application, which, of course, is measured by theclaims, nor is it intended to be limiting as to the scope of theinvention in any way.

This invention relates to a metalliding process, and particularly to aprocess for forming an extremely hard surface layer of metallic borideson a steel work piece.

In certain cases it is desirous that steel bodies be given extremelyhard surface layers so as to minimize abrasion, wear, deformation, etc.It is well known that certain of the metallic borides are extremelyhard. However, their use as a coating for steel or other ferrous metalshas heretofore been limited because of the well-known diffusioncharacteristic of ferrous ions. If a hard boride, such as chromiumboride, is plated onto a steel surface, the ferrous ions from the steelquickly diffuse into the boride layer forming ferrous borides anddestroying the desirable extreme hardness of the coating layer.

[It is, therefore, the principal object of the present invention toprovide a method for the coating of steel surfaces with extremely hardmetallic boride layers wherein means are provided for preventing thediffusion of iron from the steel body into the boride layer.

Another object is to produce on steel work pieces extremely hardmetallic borides, the major metallic constituent of which is not ironboride (Fe B) and/or (FeB).

Another object is to provide such boride coatings by utilizing the irondiffusion blocking properties in metallic carbides.

In accordance with the present invention, it has been discovered thatthe imposition of a layerof certain of the metallic carbides, such aschromium carbide, between the steel work piece and the metallic boridelayer has the effect of blocking diffusion of the ferrous ions into themetallic boride layer.

In the preferred form of the invention, the steel work piece is firstcarburized by any conventional process to diffuse carbon into the workpiece to provide a surface layer of high carbon content. Oneconventional carburizing process, as disclosed in -U.S. Pat. 2,659,685,is by packing the work piece in finely divided charcoal and heating to avery high temperature.

After carburizing, the work piece is electrolytically plated with any ofcertain nonferrous metals to diffuse said metal into the carburizedlayer to react with the carbon to form metallic carbides which act as abarrier to diffusion of the ferrous ions. The preferred nonferrous metalis chromium to form a barrier layer of chromium carbide, but it ispointed out that other carbide layers, specifically those of tantalum,titanium, vanadium, zirconium and hafnium are also included in thisdisclosure as having sutficient iron diffusion retarding properties toobtain the desired end result.

The electroplating is preferably carried out in accordance with US. Pat.No. 3,232,853 which discloses a process for chromiding wherein a cathodeof the metal to be treated and an anode of plating metal, in this casechromium, are immersed in a fluoride bath at high temperature andconnected through an external electrical circuit. With a suitabletemperature bath and suitable current passing in the system, thechromium from the anode is deposited on the cathode work piece andreacts therewith to form a surface layer of chromium carbides.

It will be noted that Pat. 3,232,853 teaches chromiding directly on ametallic base without previous carburizing. However, it is an importantdiscovery in accordance with the present invention that the formation ofa suitably impervious carbide layer, specifically chromium carbide, andmore specifically CR C6 and CR C2 cannot be obtained using the patentedmetalliding process unless carbon is present in sufficient quantities inthe base metal. Therefore, carburizing a chromided base metal work pieceis not sufficient to produce this layer, but chromiding at approximately.65 amperes per square decimeter on a carburized surface at sufficientlyhigh temperature (approximately 1000 to 1150 C.) will produce a dense,practically defect-free layer of chromium carbides. This layer isdifii'cult to obtain in thicknesses greater than ap proximately 08 mils,because of the retarding effect of the layer on the diffusion of iron inthe base metal. The diffusion of iron in the base metal is prerequisiteto the formation of layers on the base metal, as those skilled in theart are aware.

After formation of the practically impervious layer of nonferrousmetallic carbide on the work piece, the piece is then borided preferablyby the same electrolytic process. The anode of chromium or othernonferrous metal is replaced with an anode of boron and current is againcirculated through the fluoride bath at elevated temperature to deposita layer of boron on the work piece which diffuses into and reacts withthe chromium or other nonferrous metal to produce a layer of extremelyhard metallic borides the major metallic constituent of which is notiron boride or Fe B. In the case of chromium carbide layers, theprincipal boride formed is CrB it being understood that other and morecomplex borides can be pres cut. The hardness of such surface layers hasbeen found to be in the order of 4600 to 5500 DPH (diamond pyramidhardness).

In addition to electroplating, the boriding step can also be carried outby other conventional cementation processes such as are well known tothose skilled in the art.

It has been found that this method can be applied to both stainlesssteel of the straight chromium type, and to low alloy steels such as4615 and 8620, and also to the low and medium carbon plain steels suchas 1040 and 1020.

The foregoing disclosure and description of the invention isillustrative and explanatory thereof, and various changes in materialsand in details of the disclosed process may be made within the scope ofthe appended claims without departing from the spirit of the invention.

What is claimed is: l

1. A method for producing an extremely hard metallic boride layer on asteel work piece comprising the steps of (1) diffusing carbon into saidwork piece to provide thereon a layer of high carbon content,

(2) diffusing into the work piece a metal selected from: the groupconsisting of chromium, tantalum, titanium, vanadium, zirconium andhafnium to form thereon a layer of metallic carbide, and

(3) diffusing into the work piece boron to produce an extremely hardlayer of borides of said metal.

2. The method of claim 1 wherein said work piece is of stainless steeland said metal is chromium.

4 References Cited UNITED STATES PATENTS 3,029,162 4/1962 Samuel et a1.148-63 3,684,585 8/1972 Stroup et al 148l6.5 2,659,685 11/1953 Latferty1483 1.5

DOUGLAS J. DRUMMOND, Primary Examiner M. G. WITYSHYN, Assistant ExaminerUS. Cl. X.R. l48.l6.5, 31.5

